The invention relates generically to printed products obtained by the planographic printing process whereby printing inks are transferred by means of a lithographic printing plate to a printing stock, and to a process for their manufacture.
A known printing process is offset printing. Offset printing has in recent years gained considerable economic importance. In offset printing, the incompatibility of water and oil is used to divide a homogeneous oil-containing printing ink film on a printing plate into the corresponding information areas.
Offset printing plates have the properties of being hydrophilic in the non-information-bearing areas and oleophilic in the information-bearing areas. In the course of the printing process, the plate is completely wetted with a water film (dampening solution), and inking them takes place with the oil-containing printing ink. The hydrophilic areas wetted by water cannot be wetted by the hydrophobic oily ink. This produces a colored printing image on the printing plate which is transferred, via a roller mechanism, to the printing stock.
The essential advantages of offset printing are the simply, inexpensively, and very rapidly preparable print carrier, the possibility of producing printing plates directly with the aid of electronic data processing, and the possibility of four color wet-on-wet printing.
To obtain an absolutely uniform ink film over the entire width of the printing machine, the highly viscous printing inks used in offset printing need to be worked between rollers several times. Inking mechanisms of offset printing machines have up to 30 rollers between which the pasty printing inks are worked. Because of this long and thorough working, it is not possible to use, as is customary in gravure printing, low-boiling solvents but rather only high-boiling solvents since otherwise surface-drying of the printing ink on the ink rollers would already take place and, hence, disrupt the ink flow.
Solvents used in practice are high-boiling mineral oils and those vegetable oils which are hydrophobic and insoluble in water.
The use of high-boiling mineral oils as solvents in offset printing inks has considerable disadvantages. Either drying takes too long, since the mineral oils take a relatively long time to penetrate into the paper surface (strike-in) and are hampered by the water in the paper or, on increasing the drying temperature to an economically sensible level (150 to 160.degree. C.), the paper dries out and cockles, so that the subsequent processing steps, such as cutting, collating and binding, are considerably impaired. Drying, for example in rotary offset, also consumes a considerable amount of energy.
During printing, printing ink and dampening solution are mixed in offset printing, i.e., an emulsion is formed. This process is in itself advantageous, since only the thereby resulting consistency of printing ink makes possible economical four color wet-on-wet printing. As is known to an expert, lifting of the preceding printed inks from the printing stock by subsequent printing units is prevented by emulsifying water into the printing ink. On the other hand, this process has the disadvantage that on starting up a printing machine a certain time is necessary for the ink/water equilibrium to become established. It is, therefore, quite possible that, on starting up a printing machine and on intermediate switching-off and re-starting, a total of 5 to 10% of a run to be printed is obtained as spoilage.
To avoid this disadvantage, attempts have been made to emulsify water into the printing ink already in its preparation. Although this idea has been extensively propagated, it has failed to become established since these printing inks are frequently unstable. Oil and water separate and a separate damping unit is indispensible on the printing machine since the amount of water is also neither controllable nor adaptable to the paper or the printing substrate. It may be mentioned in passing that conventional printing inks can absorb about 10 to 30% of water in the printing unit on the printing machine.
In addition to the conventional offset printing method described, so-called water-less offset printing is known. This process, by means of special printing plates in which the ink-bearing layers have been rendered hydrophobic and the non-ink-bearing layers have been made extremely repellent by means of a silicone coating, attempts to print with conventional offset printing inks without dampening solution. Despite intensive development work, this printing process has failed to become widely established in the market since a four color wet-on-wet print is associated with problems similar to those in conventional relief printing. This is true in particular on pick-sensitive papers.
The state of the art of aqueous printing inks used in gravure printing and flexographic printing may be ascertained by reference to U.S. Pat. Nos. 4,374,670; 4,309,179; 3,884,707; 4,108,812, and 4,238,234, the disclosures of which are incorporated herein by reference.